Ink jet printing apparatuses having a first print cartridge for ejecting black droplets and a second print cartridge for ejecting cyan, magenta and yellow droplets are known in the art.
When hemispherical color droplets are placed side by side on a paper surface, an unintentional mixing may lead to a print defect known as "bleed." For example, a patch of yellow printed next to a patch of cyan would have a green stripe between them if ink bleed occurs. One of the solutions to bleed is to decrease the surface tension of the color inks such that rapid penetration into the paper occurs. This rapid penetration also causes the low surface tension color inks to produce larger spots than would be attained with an equivalently sized black ink droplet with less penetrating ability. This mismatch in spread factors requires that the color heating elements in the second print cartridge be much smaller than the black heating elements in the first print cartridge. The surface area of a heating element affects the size of the droplet produced when that heating element is fired.
The smaller color heating elements in the second print cartridge have the same square shape as the black heating elements in the first print cartridge. As sheet resistance is typically fixed for black and color heating elements, the resistance of the color heating elements is substantially the same as the resistance of the black heating elements.
It is generally desirable that the black and color heating elements, when fired, have substantially the same heating element energy density. If voltage pulses of substantially the same amplitude are provided to the color and black heating elements, the color heating elements must receive a much shorter firing pulse in order to keep energy density constant. Thus, a common set of drivers, i.e., a common drive circuit, which provides firing pulses of equal amplitude and duration, cannot be used to provide energy pulses to both the black and color heating elements.
In FIG. 1, heating element surface temperature-time curves are shown for a square black heating element and for a smaller, square color heating element. The superheat limit for a typical ink is shown by a dotted line. Also shown are firing pulse widths for firing pulses applied to the black and color heating elements. Because of variations in printer hardware and print cartridges, the heating elements are heated to temperatures beyond the superheat limit of the ink to ensure that ink nucleation occurs. As is apparent from these curves, the surface temperature of the smaller heating element increases at a much higher rate than that of the black heating element. This may be undesirable as it has been found that if a heating element is operated at temperatures at or above about 700.degree. C., heating element resistivity may drift downward over time. As resistivity drifts downward, the heating element will draw even more current, leading to even higher heating element surface temperatures. Unpredictable changes in heating element resistivity are to be avoided if consistent performance is to be achieved.
Thus, it would be desirable to have an ink jet printing apparatus which uses a common drive circuit to provide energy pulses to both black and color heating elements. Further, it would be desirable to have color heating elements which, when fired, do not have surface temperatures exceeding about 700.degree. C.